1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) and, more particularly, to an LCD capable of simplifying a fabrication process and improving fabrication efficiency by reducing the number of masks, and capable of reducing a wavy noise which is possibly generated when performing four masking processes.
2. Description of the Related Art
Recently, as diverse electronic devices such as a mobile phone, a PDA, a computer and a large TV, etc., are being developed, the demands for flat panel display devices that can be employed therefore are increasing.
The flat panel devices include an LCD (Liquid Crystal Display), a PDP (Plasma Display Panel), an FED (Field Emission Display), an VFD (Vacuum Fluorescent Display), etc. are being actively studied, and in terms of mass-production technologies, easiness of a driving unit and implementation of high picture quality, the LCD receives much attention.
The general LCD includes a liquid crystal panel, and the liquid crystal panel includes an array substrate, a color filter substrate attached with the array substrate with a certain gap therebetween, and a liquid crystal layer formed between the array substrate and the color filter substrate.
Liquid crystal for forming the liquid crystal layer is a material with optical anisotropy, and alignment of the liquid crystal varies according to an applied voltage to thus control light transmittance. Thus, a corresponding still image or moving picture is displayed on the LCD according to light transmittance of the liquid crystal layer.
FIG. 1 is a perspective view schematically showing the LCD according to the related art.
As shown in FIG. 1, the LCD 1 includes a color filter substrate 5, an array substrate 10, and a liquid crystal layer 30 formed between the color filter substrate 5 and the array substrate 10.
The color filter substrate 5 includes a color filter (C) including a plurality of sub-color filters 7 that implement red, green and blue colors, a black matrix 6 for dividing the sub-color filters 7 and blocking light transmission through the liquid crystal layer 30, and a transparent common electrode 8 for applying voltage to the liquid crystal layer 30.
The array substrate 10 includes gate lines 16 and data lines 17 which are IS arranged vertically and horizontally to define a plurality of pixel regions (P), TFTs, switching elements, formed at respective crossings of the gate lines 16 and the data lines 17, and pixel electrodes 18 formed on the pixel regions (P).
The color filter substrate 5 and the array substrate 10 are attached in a facing manner by a sealant (not shown) formed at an edge of an image display region to form a liquid crystal panel, and the attachment of the color filter substrates 5 and the array substrate 10 is made by an attachment key formed on the color filter substrate 5 or the array substrate 10.
FIGS. 2a to 2d are sectional views sequentially showing a fabrication process of the array substrate of the LCD in FIG. 1.
First, as shown in FIG. 2a, a gate electrode 21 made of a conductive material is formed on a substrate 10 by using a first masking process (first photolithography process).
Next, as shown in 2b, a gate insulation film 15A, an amorphous silicon thin film 24, an n+ amorphous silicon thin film 25 and a conductive metallic material 30 are sequentially deposited on the substrate 10 with the gate electrode 21 formed thereon.
Thereafter, a second masking process is performed for a selective patterning by using a half-tone mask such that a portion corresponding to the data line can be completely blocked against light and a portion corresponding to a channel region of the TFT is irradiated with a certain amount of light.
Accordingly, as shown in FIG. 2c, the amorphous silicon thin film layer is exposed, separating a source electrode 22 and a drain electrode 23, and certain portions of an active layer formed of the amorphous silicon thin film and the n+amorphous silicon thin film pattern are removed to form an ohmic contact layer 25′.
And then, a passivation film 15B is formed on the entire surface of the substrate 10 with the source and drain electrodes 22 and 23 formed thereon, and a contact hole exposing a portion of the drain electrode 23 is formed by using a third masking process.
Finally, as shown in FIG. 2d, a transparent electrode material is deposited on the entire surface so as to be connected with the drain electrode 23 via the contact hole and the pixel electrode 18 is patterned by using a fourth masking process.
After the array substrate is formed through the photography processes, it is attached with the color filter substrate to form a liquid crystal panel, and then, a backlight unit for supplying light to the liquid crystal panel is assembled to thus complete the LCD.
In this respect, however, because the active layer is formed of the amorphous silicon layer, when it receives light from the backlight unit, it becomes metal. Namely, the semiconductor has such characteristics that when it receives light and heat, its conductivity changes, so because the active layer of the liquid crystal panel is a semiconductor layer, when light is applied from the backlight unit, its conductivity increases to make the active layer metal.
Also, through the above-described processes, the active layer is formed with a line width wider than that of the data line and the source and drain electrodes, so that when the backlight unit is driven, even the active layer wider than the line width of the original data line is driven as the data line to cause an active tail phenomenon.
FIG. 3 is a sectional view showing the data line of the LCD using the 4-masking process according to the related art.
As shown, the n+ amorphous silicon layer 25 and the active layer 24 are formed at the lower portion of the data line 17, and in terms of the characteristics of an etching process, respective layers are formed to be tapered. Accordingly, the line width of the data line 17 is smaller than that of the lower active layer 24. In this respect, however, when the n+ amorphous silicon layer 25 and the active layer 24 become metal according to driving of the backlight, a region (C) corresponding to the line width of the data line 17 and the active layer 24 is driven as the data line.
As mentioned above, the related art LCD using the four masking processes has a problem that because the active layer 24 formed of the amorphous silicon becomes metal, when the backlight unit is driven, a wavy noise phenomenon occurs that wavy lines are continuously moved on a screen according to an ON/OFF operation.
In addition, in the related art, a black matrix (BM) should be formed with an additional margin of about 5 μm in order to prevent a light leakage that can be generated at an edge region (A) of the pixel electrode and a region (B) between the pixel electrode 18 and the data line 17, which causes degradation of an aperture ratio.
Moreover, because the processes of fabricating the LCD require multiple masking processes (namely, the photolithography processes), the processes are complicated and costs increase, so a method for reducing the number of masks is required.